Earthing question

On the back of your socket if there is a bar connecting the earth terminal to riveted brass inserts on the fixing screw holes then I suppose that will do. On cheaper no name sockets that is not always present in that case it is a must as a live conductor coming adrift and touching the box would leave those screws live.

When I did a lot of re-wiring I simply bought a reel of 1.5mm2 green/yellow sleeved cable and did the lot it was faster and easier than sleeving lengths of stripped cable.

It does have a bar, as it's MK.

This was to be my next question. I am minded to add the extra cable. I assume 1.5 sq mm is sufficient because it will not be carrying a continuous load for a long period. Certainly, this would be easier to fit.

I'd be tempted to install the CPC conductor from the T&E cable into the earthing terminal on the backbox, and then a second conductor from there to the socket. Means the backbox is still earthed even if the socket is removed (eg somebody fits a blanking plate). Assumed the backbox does have an earthing terminal, which older ones might not. That way you know that if a socket tester shows the socket has a good earth then the backbox does too.

The CPC needs to carry enough current to trip the circuit protection device, such as an MCB. They have a certain trip curve (must trip within so-many seconds at such-and-such current), and the CPC only needs to be able to carry current safely (without overheating) for that length of time. It shouldn't be carrying current in normal operation so it doesn't need to be sized as big as the line/neutral conductors, but it does need to be large enough to carry sufficient current to trip the MCB.

Theo

Thanks. This makes sense.

I'll do that. There is an earthing terminal.

I thought this would be the case, so thanks for confirming.

There's no need to fit a spearate fly lead to a metal backbox, so long as there is one fixed metal lug on the box. Came in in the 17th edition I think, around 15 years ago.

Interesting. The Googling I did before asking here seemed to be unclear on the subject.

It's a little more complicated than that.

Some sockets only have one earthed eyelet. If this is is not on the same side as the fixed lug on the back box you should fit the fly lead.

A few years ago someone posted on this newsgroup of a tradepack of 2g sockets that had no earthed eyelets. ISTR ToolStation were selling them but I could be wrong. Of course these do need the flylead.

Adam

The first 30 seconds of this Bigclive video shows a 2G socket with no earthed eyelets

That was me (unless someone else posted about them as well) - temporarily putting cheap sockets into my conservatory, pending replacement after decorating. I think that it may have been Screwfix, but I use both and as that would have been in about 2010, I can't remember for sure. You did well to remember that post.

For the minor effort involved, I always fit the extra earth connection.

The restraining screws may not always be in good contact.

Does it make any difference whether I use bare wire with sleeving or sleeved wire? I assume not unless I am missing something.

I do not suppose it does all the other CPCs will be sleeved so why would the fly lead be different? As a one off that is what I would do but as mentioned in my first post because I was doing a number of sockets it was worth my while getting a roll of insulated cable it was quicker and less hassle then stripping out lengths of copper conductors and then measuring and fitting sleeving.

Thanks. 'Insulated' is the word I was looking for. I think if I can get insulated cable I will use that but if not I will just extract the earth cable from a bit of twin and earth.

(Final question) Thank you for this. Could I clarify? Some say the earth wire should be continuous (folded to connect to the terminal in the box) but this is not practical for an upgrade. To my mind earthing the appliance is more important than earthing the two screws, so is there an argument to take the main earth direct to the socket with a spur to the backbox?

I suppose there is. But in that case you wouldn't be able to test that there's an earth fault with a plug-in tester, because the CPC would be fine on the socket and yet the back box could have gone live without you noticing.

While if the CPC loops through the back box you know the back box is earthed if the tester says the earth is OK.

Folding the CPC wire through is good because you can ensure connectivity to the end even if the terminal in the middle is poorly connected, but as you say that's hard in a retrofit.

An argument against looping through is if there's no RCD on the circuit: adding the backbox terminal in series would increase the loop impedance and thus the chances of not taking enough current to trip the MCB. But that's not an issue on an RCD protected circuit, where it only needs 30mA L to E to trip.

So on a modern CU as long as the earthing system is able to carry 30mA it's enough to trip the protective device, which would likely be satisfied by even badly made connections that aren't completely open circuit.

Theo

Thanks. It's RCBO (same principle I assume) so I shall follow your original advice.

I dunno why, but that sounds sketchy to me.

If you have a high impedance to earth would that not affect the RCDs ability to sense fault current?